This invention relates to cuprate-based superconducting bodies, a powder that can be used for their production, and the use of such bodies.
The term xe2x80x9csuperconducting bodies based on cuprate materialxe2x80x9d in this application denotes all those oxide ceramics (in the form of molded bodies, films, applied to strip or substrates, wire, xe2x80x9cpowder-in-tube,xe2x80x9d or used as the target in a coating process), which contain CuO and have superconducting properties at a sufficiently low temperature.
Superconducting bodies, e.g., molded bodies, can be used, for instance, in cryomagnetic applications in relatively high external magnetic fields. They can, for example, be superconducting wires or components in electric motors. As a function of the strength of the external magnetic field it has been observed that the drop in the critical current density is all the more pronounced the greater the external magnetic field is.
It is the object of the present invention to provide superconducting bodies based on cuprate material with an increased critical current density in the presence of external magnetic fields.
This and other objects have been attained in accordance with the present invention by providing a superconducting molded body based on a cuprate material and obtained by melt texturing, wherein said body has a zinc cation content of 50 to 5000 ppm by weight.
In accordance with a further aspect of the invention, the object of the invention have been attained by providing a cuprate powder for producing superconducting material, said powder having a zinc cation content in an amount of 50 ppm to 1000 ppm by weight and a grain size distribution of d90% of less than 35 xcexcm determined by a CILAS laser granulometer.
The superconductive body of the present invention may be formed into various superconductive articles including, e.g., a superconducting cable, permanent magnet, short circuit current limiter, transformer, generator, SMES, flywheel energy storage system, high-field magnet, electromagnet or superconducting magnetic bearing.
The superconducting bodies based on cuprate material according to the invention are characterized by a zinc cation content of 50 to 5000 ppm. The zinc is typically present in the form of the oxide. Preferred are bodies with a zinc-cation content ranging from 100 to 1000 ppm. This weight range preferably refers to the superconducting phase in the body (auxiliary material, such as special additives, fillers, targets and any interlayers, other substrates or (silver) tubes in powder-in-tube bodies are then not included in the calculation).
Bodies based on cuprate material with the inventive zinc cation content generally have the advantages of the invention. A preferred cuprate material is a cuprate material of the rare earth metal alkaline-earth metal cuprate type, particular yttrium barium cuprate as well as cuprate material of the bismuth (lead) alkaline-earth metal copper oxide type. These materials are known per se; well-suited materials have already been mentioned above. Bismuth strontium calcium cuprate with an atomic ratio of 2:2:1:2 and 2:2:2:3 is especially suitable. In the latter, a portion of the bismuth may be replaced with lead. Bismuth strontium calcium cuprates with modifications in the stoichiometry of the aforementioned atomic ratios can of course also be used.
Superconducting cuprate material and the manner of producing it (e.g., film formation, melt texturing, etc.) are known per se.
Particularly suitable, for instance, are the rare earth metal alkaline-earth metal cuprates described in WO 88/05029, especially YBa2Cu3O7xe2x88x92x (xe2x80x9cYBCOxe2x80x9d), bismuth (lead) alkaline-earth metal cuprates, such as bismuth strontium calcium cuprates and bismuth lead strontium calcium cuprates, especially of the 2212-type (Bi:Sr:Ca:Cu=2:2:1:2) and of the 2223-type (Bi:Sr:Ca:Cu=2:2:2:3). Here a portion of the Bi may be replaced with lead. Bi-containing cuprates are described, for example, in EP 336,450; U.S. Pat. No. 4,990,488 (=DE 37 39 886); U.S. Pat. No. 5,145,831 (=EP 330,214); U.S. Pat. No. 4,880,771 (=EP 332,291) and EP 330,305.
The conversion of the raw materials (metal oxides or carbonates) into a superconducting powder is known.
German patent DE 42 16 545 discloses such a process. In a multistage heat treatment the material is heated to a temperature of 950xc2x0 C. and is then cooled again.
Various types of bodies may be produced, e.g., molded bodies, particularly by melt texturing. Park et al., U.S. Pat. No. 6,063,735 (=WO 97/06567) discloses a yttrium barium cuprate mixture, which is particularly suitable for the production of melt-processed high temperature superconductors with high levitation force. Important in this mixture is that it contains less than 0.6% by weight of free copper oxide that is not bound in the yttrium barium cuprate phase and less than 0.1% by weight carbon. In the melt texturing process, additives are added, which form xe2x80x9cpinningxe2x80x9d centers or promote their formation. These centers permit an increase in the critical current density in the superconductor. Additives enhancing flux pinning are, for instance Y2BaCuO5, Y2O3, PtO2, Ag2O, CeO2, SnO2, ZrO2, BaCeO3 and BaTiO3. These additives may be added in amounts ranging from 0.1 to 50% by weight, where the yttrium barium cuprate powder is set at 100% by weight. Platinum oxide, for instance, is advantageously used in an amount of 0.5 to 5% by weight.
Other bodies, for instance, are thin films, see EP 354,616 (=DE 38 26 924) (deposition from a homogenous solution), thick films in the form of strip or wire with interlayer by calcination of a precursor phase deposited on the substrate, see U.S. Pat. No. 5,096,878 (=EP 339,801), film deposition by PVD process, see U.S. Pat. No. 4,988,670 (=EP 299,870), CVD process, see U.S. Pat. No. 5,140,003 (=EP 388,754), and wire in the form of a ceramic powder-filled metal tube (powder-in-tube technology), see U.S. Pat. No. 5,075,285 (=DE 37 31 266).
EP 375,134 discloses a glass-ceramic molded body, while U.S. Pat. No. 5,047,391 (=EP 362,492) describes a casting solidified from the melt.
Zinc cations have proven to be of crucial importance. Those superconducting materials, e.g., made of rare earth metal alkaline-earth metal cuprate, which do not already contain strontium, calcium and/or aluminum as lattice components (e.g., in Bi, Pbxe2x80x94Srxe2x80x94Ca cuprates), advantageously also contain other foreign metal ions through Zn zinc cations, namely strontium, calcium and/or aluminum. These foreign metal ions are ions of other metals besides those that are added as flux pinning additives as described above.